Actuator used with an optical pickup

ABSTRACT

An actuator for an optical pickup having a base on which a holder is fixed, a bobbin, a support member, and a magnetic circuit. Pluralities of installation holes are formed in the bobbin such that a plurality of objective lenses to record and/or reproduce optical discs having different recording densities are installed at different heights. The support member has one end coupled to the bobbin and the other end coupled to the holder movably supports the bobbin. The magnetic circuit drives the bobbin in focusing and tracking directions. The magnetic circuit is divided into a first magnetic circuit to drive the objective lenses in the focusing direction, and a second magnetic circuit to drive the objective lenses in the tracking direction, thereby reducing the weight of a moving portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 10/247,794 filed on Sep. 20, 2002 now U.S. Pat. No. 6,895,593 in theU.S. patent and Trademark Office. This application claims the benefit ofKorean Application No. 2001-75671, filed Dec. 1, 2001, in the KoreanIndustrial Property Office, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an actuator used with an opticalpickup, and more particularly, to an actuator used with an opticalpickup having a plurality of objective lenses so that a plurality ofoptical discs having different recording densities can be recordedand/or reproduced.

2. Description of the Related Art

It is well-known that digital versatile discs (DVDs) are recorded and/orreproduced using light having a wavelength of 650 nm (or 635 nm) andusing an objective lens having a numeral aperture of 0.6 (0.65 in thecase of a recordable disc). In the case of DVDs having a diameter of 120mm and a track pitch of 0.74 μm, DVDs have a capacity of 4.7 or moregigabytes for a single surface. Accordingly, DVDs are not suitable for arecording medium on which high definition (HD) moving pictureinformation is to be recorded. This is the reason that a recordingcapacity of 23 or more gigabytes for a single surface is required sothat 135-minute HD moving picture information can be recorded on therecording medium.

To meet the need of a high density recording capacity, high densityoptical discs, i.e., next generation DVDs (hereinafter, highdefinition-digital versatile discs (HD-DVDs)) using light having awavelength shorter than that of a red color, i.e., blue light, and anobjective lens having a numerical aperture greater than 0.6 and having amore narrow track have been developed and standardized.

In order to obtain an adequate tolerance with respect to tilt of opticaldiscs, as the numerical aperture of an objective lens increases for highdensity, the thickness of optical discs should be reduced. Consideringallowable tolerance with respect to the tilt of optical discs, the 1.2mm thickness of compact discs (CDs) is reduced to 0.6 mm for DVDs, andthere are further possibilities that the thickness of HD-DVDs can bereduced to 0.1 mm. Regarding the numerical aperture of an objectivelens, the 0.45 numerical aperture of CDs is increased to 0.6 for DVDs,and there are further possibilities that the numerical aperture forHD-DVDs can be increased to 0.85. Also, considering a recordingcapacity, there are good possibilities of adopting a light source suchas a celadon light source for HD-DVDs. In the development of opticaldiscs with new specifications, compatibility with existing optical discsis important.

However, a special technique is required to design and manufacture anobjective lens having a high numerical aperture of about 0.85 as asingle lens. Besides, it is difficult to make the working distance of anobjective lens having a high numerical aperture be as long as that of anobjective lens for DVDs.

Accordingly, to solve the working distance problem, in a compatibleoptical pickup in which optical discs having high densities can berecorded and/or reproduced, at least one objective lens used to recordand/or reproduce CDs and/or DVDs, and an objective lens for high densityrecording having a higher numerical aperture than the previous objectivelens should be separately provided.

For driving in both directions, i.e., a focusing direction and atracking direction, an actuator for an optical pickup has a magneticcircuit. The actuator maintains an interval between an optical disc andan objective lens in the focusing direction and moves the objective lensin a desired track position (center of a track). As described above,however, the above-mentioned optical pickup adopting a plurality ofoptical discs having different recording densities requires objectivelenses corresponding to the plurality of optical discs having differentrecording densities, and thus an actuator used with an optical pickuphaving a plurality of objective lenses loads a plurality of objectivelenses on a moving portion so as to move in the focusing and trackingdirections.

Referring to FIG. 1, an actuator disclosed in Japanese PatentPublication No. Hei 10-106001 includes a lens holder (bobbin: 7) atwhich two objective lenses 2 and 3 are installed, a rotation axis 1which rotatively guides the lens holder 7, and a magnetic circuit whichdrives the objective lenses 2 and 3 in focusing and tracking directions.

The two objective lenses 2 and 3 are arranged at a predetermined angle θwith respect to the center of the rotation axis 1 and rotatesimultaneously at the predetermined angle θ with respect to the rotationaxis 1.

The magnetic circuit comprises focusing and tracking coils 8 and 9installed at the lens holder 7, yokes 5 a and 5 b, and a plurality ofmagnets 6. The plurality of magnets 6 are arranged at an angle greaterthan the predetermined angle of the objective lenses 2 and 3 withrespect to the rotation axis 1. The tracking coil 9 is provided tocorrespond to the magnets 6.

In a conventional actuator having the above structure as illustrated inFIG. 1, the two objective lenses 2 and 3, arranged at the predeterminedangle θ with respect to the rotation axis 1 within one lens holder 7,rotates simultaneously by the predetermined angle θ with the respect tothe rotation axis 1 such that a plurality of optical discs havingdifferent recording densities and thicknesses can be recorded and/orreproduced. Detailed descriptions of the conventional actuatorillustrated in FIG. 1 and disclosed in Japanese Patent Publication No.Hei 10-106001 will be omitted.

Since the above conventional actuator requires a complicated magneticcircuit to control the positions of the objective lenses 2 and 3 beforeand after the objective lenses 2 and 3 rotate simultaneously by thepredetermined angel θ with respect to the rotation axis 1, and anadditional structure in which the objective lenses 2 and 3 rotatesimultaneously by the predetermined angel θ with respect to the rotationaxis 1, the entire structure of the conventional actuator iscomplicated.

Since the above conventional actuator presents an axis perturbation typedesign, sensitivity is relatively low and non-linear properties arerelatively large, the above conventional actuator is less suitable torecord and/or reproduce high density optical discs than DVDs.

Moreover, in the above conventional actuator, compared to the structureof an existing actuator in which one objective lens is installed in abobbin, the moving portion of the above conventional actuator is large,and heavier, and thus the actuator sensitivity is further degraded.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anactuator used with an optical pickup having an improved structure, moresimple than the entire structure of a conventional actuator for anoptical pickup, in which two objective lenses are installed in onebobbin, the weight of a moving portion of the actuator is reduced andthe actuator sensitivity is improved.

Additional objects and advantages of the invention will be set forth inpart in the description which follows and, in part, will be obvious fromthe description, or may be learned by practice of the invention.

The foregoing and other objects of the present invention are achieved byproviding an actuator for an optical pickup including: a base on which aholder is fixed; a bobbin in which a plurality of installation holes areformed such that a plurality of objective lenses to record and/orreproduce optical discs having different recording densities areinstalled at different heights; a support member, which movably supportsthe bobbin, having one end coupled to the bobbin and the other endcoupled to the holder; and a magnetic circuit which drives the bobbin infocusing and tracking directions.

In an aspect of the present invention, the plurality of installationholes are arranged in a direction corresponding to the direction of aradius of the optical disc.

In another aspect of the present invention, the magnetic circuit isdivided into a first magnetic circuit to drive the objective lenses inthe focusing direction, and a second magnetic circuit to drive theobjective lenses in the tracking direction, thereby reducing the weightof a moving portion of the actuator.

The foregoing and other objects of the present invention may also beachieved by providing an actuator for an optical pickup including: abase on which a holder is fixed; a bobbin in which a plurality ofinstallation holes are formed such that a plurality of objective lensesto record and/or reproduce optical discs having different recordingdensities are installed; a support member, which movably supports thebobbin, having one end coupled to the bobbin and the other end coupledto the holder; and a magnetic circuit which drives the bobbin infocusing and tracking directions, the magnetic circuit being dividedinto a first magnetic circuit to drive the objective lenses in thefocusing direction, and a second magnetic circuit to drive the objectivelenses in the tracking direction, thereby reducing the weight of amoving portion of the actuator.

It is an aspect of the present invention that the plurality ofinstallation holes are provided such that the plurality of objectivelenses are installed at different heights. It is another aspect of thepresent invention that the plurality of installation holes are arrangedin a direction corresponding to the direction of a radius of the opticaldisc. It is yet another aspect of the present invention that the firstmagnetic circuit includes a focusing coil installed in the bobbin and afocusing magnet installed on the base opposite to the focusing coil, andthe second magnetic circuit includes a tracking coil installed in thebobbin and a tracking magnet installed on the base opposite to thetracking coil.

It is yet another aspect of the present invention that the focusingmagnet is a two-pole magnetized polarization magnet, the poles beingdisposed in the focusing direction, and the focusing coil is provided tocorrespond to the focusing magnet.

Alternatively, wherein the tracking magnet is a three-pole magnetizedpolarization magnet, the poles are disposed in the tracking direction,and two tracking coils are provided to correspond to the trackingmagnet.

It is yet another aspect of the present invention that the first andsecond magnetic circuits are installed at sides of the bobbin parallelwith a direction corresponding to the direction of a radius of theoptical disc. It is yet another aspect of the present invention that thefirst magnetic circuit further includes at least one yoke of an internalyoke and an external yoke to guide magnetic flux generated in thefocusing magnet. It is yet another aspect of the present invention thatthe tracking magnet and the tracking coil of the second magnetic circuitare arranged closer to the center of the bobbin than the focusing coiland the focusing magnet of the first magnetic circuit. It is yet anotheraspect of the present invention that the plurality of installation holesinclude a first installation hole in which a first objective lens for atleast one kind of low density optical disc is installed, and a secondinstallation hole in which a second objective lens for a high densityoptical disc is installed.

It is yet another aspect of the present invention that the secondinstallation hole is located in a further inside diameter of the opticaldisc than the first installation hole. It is yet another aspect of thepresent invention that the low density optical disc is at least oneselected from a CD-family optical disc and a DVD-family optical disc,and the high density optical disc has a higher density than theDVD-family optical disc.

It is yet another aspect of the present invention that when a workingdistance of the first objective lens for a low density optical discinstalled in the first installation hole is represented as WD1, and aworking distance of the second objective lens for a high density opticaldisc installed in the second installation hole is represented as WD2,the first and second installation holes should be provided so that thefirst and second objective lenses can be installed to satisfy thefollowing Equation:WD1≧WD2separation distance with respect to optical disc of second objectivelens=WD2+αwhere, α=|WD1−WD2|×(0.1˜1.0).and contact between the second objective lens having a shorter workingdistance and the optical disc can be prevented when the optical disc isinstalled and the first objective lens having a longer working distanceoperates.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a schematic view of a conventional actuator;

FIG. 2 is a schematic perspective view of the entire structure of anactuator used with an optical pickup according to an embodiment of thepresent invention;

FIG. 3 is a top view of the actuator of FIG. 2;

FIG. 4 is a perspective view of the bobbin illustrated in FIG. 2;

FIG. 5 is a partial cross-sectional view taken along line V—V of FIG. 4;

FIG. 6 illustrates a separation distance between an optical disc and twoobjective lenses when the two objective lenses having different workingdistances are installed in an existing actuator for an optical pickup;

FIG. 7 illustrates a separation distance between an optical disc and twoobjective lenses when the two objective lenses having different workingdistances are installed in the actuator used with an optical pickupaccording to an embodiment of the present invention;

FIG. 8 is a schematic perspective view of a first magnetic circuitadopted in the actuator used with an optical pickup according to FIG. 2;

FIGS. 9A and 9B illustrate a principle of driving the bobbin in afocusing direction by the first magnetic circuit of FIG. 8;

FIG. 10 is a schematic perspective view of a magnetic circuit adopted inthe actuator used with an optical pickup of FIG. 2;

FIGS. 11A and 11B illustrate a principle of driving the bobbin in afocusing direction by the magnetic circuit of FIG. 10;

FIG. 12 is a schematic perspective view of another embodiment of themagnetic circuit of FIG. 10 adopted in the actuator used with an opticalpickup according to FIG. 2; and

FIG. 13 is a perspective view of a focusing magnet and internal andexternal yokes to guide a magnetic flux generated by the focusing magnetillustrated in FIG. 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

An actuator used with an optical pickup according to an embodiment ofthe present invention is a two-axis driving apparatus in which aplurality of objective lenses having different working distances areinstalled in one bobbin, and a moving portion of the actuator movesindependently in focusing and tracking directions. The actuator usedwith an optical pickup according to an embodiment of the presentinvention can be adopted in optical pickups to record and/or reproducetwo kinds of optical discs, such as HD-DVD and DVD, having differentrecording densities, or in optical pickups to record and/or reproducethree or more kinds of optical discs, such as HD-DVD, DVD, and CD,having different recording densities.

FIG. 2 is a schematic perspective view of the entire structure of anactuator for an optical pickup according to an embodiment of the presentinvention, FIG. 3 is a top view of FIG. 2, FIG. 4 is a perspective viewof the bobbin illustrated in FIG. 2, and FIG. 5 is a cross-sectionalview taken along line V—V of FIG. 4.

Referring to FIGS. 2 through 5, the actuator for an optical pickupaccording to an embodiment of the present invention includes a base 10having a holder 11 installed at one side of the base 10, a bobbin 20 onwhich a plurality of installation holes 21 and 25 are formed so that aplurality of objective lenses 31 and 35 having different workingdistances are installed on the bobbin 20, a support member 13(suspension), which movably supports the bobbin 20, having one endcoupled to the bobbin 20 and the other end coupled to the holder 11, anda magnetic circuit which drives the bobbin 20 in focusing and trackingdirections.

The plurality of objective lenses 31 and 35 includes a first objectivelens 31 used to record and/or reproduce at least one kind of low densityoptical disc having different recording densities, and a secondobjective lens 35 used to record and/or reproduce higher density opticaldiscs (hereinafter, high density optical discs) than the low densityoptical discs. The first and second objective lenses 31 and 35 havedifferent working distances. For example, the first objective lens 31can record and/or reproduce DVD-family optical discs (hereinafter,DVDs), as low density optical discs, and can additionally record and/orreproduce CD-family optical discs (hereinafter, CDs). Also, for example,the second objective lens 35 can record and/or reproduce HD-DVD familyoptical discs (hereinafter, HD-DVDs) as higher density optical discsthan DVDs. Here, the plurality of objective lenses 31 and 35 maycomprise three or more objective lenses having different workingdistances so that the plurality of objective lenses 31 and 35 can beused to record and/or reproduce three or more kinds of optical discshaving different recording densities.

The actuator according to an embodiment of the present invention isprovided so that the plurality of objective lenses 31 and 35 can beinstalled in one bobbin 20 in a direction (R direction) corresponding tothe direction of a radius of an optical disc, and the actuator can becompatible with an optical pickup which requires a plurality ofobjective lenses. This is the reason that the optical pickup recordsand/or reproduces an information signal while moving in the direction ofthe radius of the optical disc in an optical disc drive.

The actuator used with an optical pickup according to an embodiment ofthe present invention is provided so that the second objective lens 35for a high density optical disc is installed in a more inner diameter ofthe optical disc than the first objective lens 31 for a low densityoptical disc. This is the reason that a region in which data recordingand/or reproduction of an HD-DVD starts in a more inner diameterlocation than a region in which data recording and/or reproduction of aDVD of a low density optical disc starts.

As described above, when the actuator used with an optical pickupaccording to an embodiment of the present invention has a structure inwhich the two objective lenses 31 and 35 are installed in the bobbin 20,as illustrated in FIG. 4, a first installation hole 21 in which thefirst objective lens 31 is installed, and a second installation hole 25in which the second objective lens 35 is installed are formed in thebobbin 20. Here, the number of installation holes formed in the bobbin20 corresponds to the number of objective lenses to be installed.

The first and second installation holes 21 and 25 are arranged in thedirection R, as illustrated in FIGS. 4 and 5, and are provided so thatthe first and second objective lenses 31 and 35 are installed atdifferent heights with respect to each other. Specifically, aninstallation projection 21 a is formed in the first installation hole 21at a relatively deep position from an upper side opposite to the opticaldisc of the bobbin 20 such that the first objective lens 31, having alonger working distance for a low density optical disc, can beinstalled. An installation projection 25 a is formed in the secondinstallation hole 25 at the same height as an upper side opposite to theoptical disc of the bobbin 20 (or at a position closer to the upper sideof the bobbin 20 than the first installation projection 21 a formed inthe first installation hole 21) such that the second objective lens 35,having a shorter working distance for a high density optical disc, canbe installed therein.

When a working distance of the first objective lens 31 for a low densityoptical disc installed in the first installation hole 21 is representedas WD1, and a working distance of the second objective lens 35 for ahigh density optical disc installed in the second installation hole 25is represented as WD2, the first and second installation holes 21 and 25should be provided so that the first and second objective lenses 31 and35 can be installed to satisfy the following equation:WD1≧WD2separation distance with respect to optical disc of second objectivelens=WD2+αHere, α=|WD1−WD2|×(0.1˜1.0).  Equation 1:

FIG. 6 illustrates a separation distance between an optical disc 50 andthe first and second objective lenses 31 and 35 when the first andsecond objective lenses 31 and 35, having different working distances,are installed in an existing actuator for an optical pickup. FIG. 7illustrates a separation distance between the optical disc 50 and thefirst and second objective lenses 31 and 35 when the first and secondobjective lenses 31 and 35, having different working distances, areinstalled in the bobbin 20 of the actuator used with an optical pickupaccording to an embodiment of the present invention. As seen from thecomparison of FIG. 6 and FIG. 7, if the first and second installationholes 21 and 25 are formed so that the first and second objective lenses31 and 35 can be installed while satisfying the above Equation 1, abasic separation distance between the second objective lens 35 and theoptical disc 50 is WD2+α. Thus, contact (interference) between thesecond objective lens 35, having a shorter working distance, and theoptical disc 50 can be prevented when the optical disc 50 is initiallyinstalled, and when the first objective lens 31 having a longer workingdistance operates.

Referring back to FIGS. 2 and 3, in the actuator used with an opticalpickup according to an embodiment of the present invention, the magneticcircuit is divided into a first magnetic circuit 51 to drive the firstand second objective lenses 31 and 35 in the focusing direction, and asecond magnetic circuit 55 to drive the first and second objectivelenses 31 and 35 in the tracking direction, thereby reducing the weightof the moving portion. Here, the first and second magnetic circuits 51and 55 are installed at the same side of the bobbin 20 (preferably, at aside parallel with the direction R).

Here, in an optical pickup assembly when the first and second objectivelenses 31 and 35 are installed in the bobbin 20 of the actuatoraccording to an embodiment of the present invention, the moving portionincludes the bobbin 20, the first and second objective lenses 31 and 35,and a magnetic circuit portion (preferably, focusing and tracking coils53 and 57) installed in the bobbin 20.

The first magnetic circuit 51 comprises the focusing coil 53 and afocusing magnet 52. The focusing coil 53 is installed at both sides ofthe bobbin 20, parallel with the direction R, and the focusing magnet 52is installed on the base 10 to be opposite to the focusing coil 53.

As illustrated in FIG. 8, the focusing magnet 52 is a two-polemagnetized polarization magnet, and the focusing coil 53 is formed in arectangular shape in which long sides of the focusing coil 53 are put onthe N-pole portion 52 a and S-pole portion 52 b of the focusing magnet52. In this case, as illustrated in FIGS. 9A and 9B, portionscorresponding to a pair of long sides of the focusing coil 53 are theeffective focusing coil which contributes to generate an electromagneticforce. Likewise, when the portions corresponding to a pair of long sidesof the focusing coil 53 having a rectangular shape act as an effectivefocusing coil, the overall length of the focusing coil 53 installed inthe moving portion of the actuator can be reduced.

Here, as the N-pole portion 52 a of the focusing magnet 52 is placed onthe upper side of the S-pole portion 52 b of the focusing magnet 52 andthe magnetic field from the N-pole portion 52 a protrudes from thepaper, as illustrated in FIG. 9A, when current flows counterclockwisethrough the focusing coil 53, a force is applied in the upper directionof the portions corresponding to a pair of long sides of the focusingcoil 53, according to Flemings' left hand rule. When the current flowsclockwise through the focusing coil 53, as illustrated in FIG. 9B, theforce is applied in the lower direction of the portions corresponding toa pair of long sides of the focusing coil 53. Thus, when the polarityand amount of the current flowing through the focusing coil 53 areadjusted, the position in the focusing direction of the first and secondobjective lenses 31 and 35 installed in the bobbin 20 can be controlled.

The second magnetic circuit 55 comprises the tracking coil 57 and atracking magnet 56. In this case, the tracking coil 57 is installed atboth sides of the bobbin 20, and the tracking magnet 56 is installed onthe base 10 to be opposite to the tracking coil 57.

As illustrated in FIG. 10, the tracking magnet 56 is a two-polemagnetized polarization magnet in the direction R, and the tracking coil57 is formed in a rectangular shape in which long sides of the trackingcoil 57 are put on the N-pole portion 56 a and S-pole portion 56 b ofthe tracking magnet 56. In this case, as illustrated in FIGS. 11A and11B, portions corresponding to a pair of long sides of the tracking coil57 are the effective tracking coil which contributes to generate anelectromagnetic force. Likewise, when the portions corresponding to apair of long sides of the tracking coil 57 having a rectangular shapeact as an effective focusing coil, the overall length of the trackingcoil 57 installed in the moving portion of the actuator can be reduced.

Here, when the N-pole portion 56 a of the tracking magnet 56 is placedon the left side of the S-pole portion 56 b of the tracking magnet 56,and the magnetic field from the N-pole portion 56 a protrudes frompaper, as illustrated in FIG. 11A, when current flows counterclockwisethrough the tracking coil 57, the force is applied in the left directionof the portions corresponding to a pair of long sides of the trackingcoil 57. When the current flows clockwise through the tracking coil 57,as illustrated in FIG. 11B, the force is applied in the right directionof the portions corresponding to a pair of long sides of the trackingcoil 57. Thus, when the polarity and amount of the current flowingthrough the tracking coil 57 are adjusted, the position in the trackingdirection of the first and second objective lenses 31 and 35 installedin the bobbin 20 can be controlled.

Alternatively, as illustrated in FIG. 12, a three-pole magnetizedpolarization magnet in the direction R may be used as the trackingmagnet 56. Two tracking coils 57 are provided so that both long sides ofthe tracking coils 57 are put on the N-pole portion 56 a and S-poleportions 56 b of the tracking magnet 56. Here, in the arrangementbetween the three-pole magnetized tracking magnet 56 and the twotracking coils 57, the direction of the force applied to the trackingcoils 57 can be determined from FIGS. 11A and 11B according to thedirection of the current flowing through the tracking coils 57, and thusdetailed descriptions thereof will be omitted.

Consequently, since the magnetic circuit can increase the length of theeffective focusing coil and the length of the effective tracking coil,the overall lengths of the focusing coil 53 and the tracking coil 57 canbe reduced, and thus the weight of the moving portion of the actuatorcan be reduced.

The actuator used with an optical pickup according to this embodiment ofthe present invention further includes an internal yoke 58 and anexternal yoke 59 so that the intensity of an effective magnetic field togenerate a driving force in the focusing direction can be increased byguiding a magnetic flux generated in the focusing magnet 52.

FIG. 13 is a perspective view of the focusing magnet 52 and internal andexternal yokes 58 and 59 to guide a magnetic flux generated in thefocusing magnet 52 illustrated in FIG. 2. The internal and externalyokes 58 and 59 may be formed monolithically of the same material asthat of the base 10. Here, one of the internal and external yokes 58 and59 may be included so as to guide the magnetic flux generated in thefocusing magnet 52.

As described above, when the actuator used with an optical pickupaccording to this embodiment of the present invention further includesthe internal and external yokes 58 and 59, the focusing magnet 52 isinstalled at one side facing the bobbin 20 of the external yoke 59, andthe internal yoke 58 is located between the focusing coil 53 and thecenter of the bobbin 20. Thus, as illustrated in FIG. 4, an insertionhole 61, into which the internal yoke 58 is inserted, is furtherincluded in the bobbin 20.

As illustrated in FIGS. 2 and 13, the internal yoke 58 may be used as amount for the tracking magnet 56 of the second magnetic circuit 55. Inthis case, the tracking magnet 56 is installed at a side facing thecenter of the bobbin 20 of the internal yoke 58. The tracking coil 57 isinstalled in the bobbin 20 to be opposite to the tracking magnet 56 inthe insertion hole 61.

As described above, when the actuator used with an optical pickupaccording to this embodiment of the present invention includes theinternal and external yokes 58 and 59, and the internal yoke 58 is usedas a mount to install the tracking magnet 56, and the tracking coil 57is arranged at a side toward the center of the bobbin 20 of theinsertion hole 61, the insertion hole 61 has a size in which movement inthe focusing and tracking directions of the bobbin 21 is not affected bythe internal yoke 58, the tracking magnet 56, and the tracking coil 57located in the insertion hole 61. Here, when the bobbin 20 largelydeviates from a desired position, the bobbin 20 is suspended by theinternal yoke 58 inserted in the insertion hole 61, and the movement ofthe bobbin 20 is limited, and thus the internal yoke 58 guides themovement of the bobbin 20.

As described with reference to FIGS. 2 and 13, if the intensity of theeffective magnetic field is maximized while providing the internal andexternal yokes 58 and 59, and the focusing coil 53 is formed in arectangular shape such that both long sides of focusing coil 53 are puton the N-pole and S-pole portions 52 a and 52 b of the focusing magnet52 while providing a two-pole magnetized polarization magnet, i.e., thefocusing magnet 52, the poles being disposed in the focusing direction,the length of the focusing coil 53 to generate a magnetic driving forceof a desired size, and the corresponding occupied volume can be reduced.Also, if the tracking coil 57 is formed in a rectangular shape such thatboth long sides of the tracking coil 57 are put on the N-pole and S-poleportions 56 a and 56 b of the tracking magnet 56 while providing atwo-pole or three-pole magnetized polarization magnet, the poles beingdisposed in the direction R, the length of the tracking coil 57 togenerate a magnetic driving force of a desired size, and thecorresponding occupied volume can be reduced. Consequently, the weightof the moving portion can be reduced greatly by the structure of themagnetic circuit using the polarization magnets as the focusing andtracking magnets 52 and 56.

In the actuator used with an optical pickup according to the embodimentsof the present invention, since the two objective lenses 31 and 35 areinstalled in the bobbin 20, the weight of the objective lenses 31 and 35installed in the bobbin 20 is increased compared with an existingactuator in which one objective lens is installed in one bobbin.However, in the actuator used with an optical pickup according to theembodiments of the present invention, since the magnetic circuit isdivided into the first magnetic circuit 51 to move the magnetic circuitin the focusing direction and the second magnetic circuit 55 to move themagnetic circuit in the tracking direction, and the polarization magnetsprovide the focusing and tracking magnets 52 and 56, the weight of themagnetic circuit (in particular, the focusing coil 53 and the trackingcoil 57) installed in the moving portion of the actuator can be reducedas compared with the existing actuator, and moreover, theelectromagnetic force to move in the focusing and tracking directions isgreater than that of an existing actuator. Even though the movingportion of the actuator used with an optical pickup according to theembodiments of the present invention is heavier than that of an existingactuator in which only one objective lens is installed in the bobbin,reduction in sensitivity can be prevented. Also, the weight of themoving portion of the actuator used with an optical pickup according tothe embodiments of the present invention may be not heavier than that ofthe existing actuator in which only one objective lens is installed inthe bobbin.

Although the actuator used with an optical pickup according to theembodiments of the present invention is described and illustrated as astructure in which the two objective lenses 31 and 35 having differentworking distances are installed in one bobbin 20, this is only anexample, and the present invention is not limited thereto. That is, theactuator used with an optical pickup according to the embodiments of thepresent invention has a structure in which two or more objective lensesare installed in one bobbin, and thus can be adopted in the opticalpickup such that two or three or more kinds of optical discs havingdifferent recording densities, such as CDs, DVDs, and HD-DVDs can becompatible with and recorded and/or reproduced.

The optical pickup in which the above-mentioned actuator according tothe embodiments of the present invention can be adopted may have astructure in which an optical system to record and/or reproduce highdensity optical discs, such as HD-DVD, and an optical system to recordand/or reproduce low density optical discs, such as DVDs (andadditionally, CDs), are separately included. Specific examples of anoptical structure of the optical pickup adopting the actuator accordingto the embodiments of the present invention can be understood by peopleskilled in the art, and thus detailed descriptions and illustrations ofspecific examples of the optical structure of the optical pickupadopting the actuator according to the embodiments of the presentinvention will be omitted.

Hereinafter, the operation of an optical disc drive in which theactuator used with an optical pickup according to an embodiment of thepresent invention is adopted will be described.

If the optical disc is inserted into the optical disc drive, the type ofoptical disc is determined by a photodetector installed in the opticalpickup or a separate detection device. The type of optical discs isdiscriminated according to a recording density.

When the discrimination of the type of the optical disc is completed,focusing and tracking servos for an objective lens corresponding to thetype of the optical disc sequentially operate. That is, if the insertedoptical disc is a high density optical disc (for example, a HD-DVD), thefocusing and tracking servos operate such that the objective lenses,arranged in an inner diameter of the bobbin, are located in a properposition of the optical disc. Also, if the inserted optical disc is alow density optical disc (for example, a DVD or CD), the focusing andtracking servos operate such that the objective lenses, arranged in anouter diameter of the bobbin, are located in a proper position of theoptical disc.

Here, the focusing and tracking servos refer to focusing and trackingerror signals detected by the photodetector installed in the opticalpickup, allowing current to flow through the focusing and tracking coilsattached to the moving portion of the actuator, thereby generating thedisplacement of the moving portion. When the current flows through thefocusing and tracking coils, an electromagnetic force is generated bythe interaction between the current flowing through the focusing andtracking coils and the magnetic flux generated in the focusing magnetand the tracking magnet such that the bobbin is moved in the focusingand tracking directions.

As described above, the actuator used with an optical pickup accordingto the embodiments of the present invention has a structure in which twoobjective lenses are installed in one bobbin, and a device required torotate the objective lenses simultaneously by a predetermined angle withrespect to the rotation axis, unlike with a conventional actuator topresent an axis perturbation type, is not required, and thus the entirestructure of the actuator is simplified.

Also, the actuator used with an optical pickup according to theembodiments of the present invention has a structure in which a magneticcircuit to adjust the bobbin in the tracking direction, and a magneticcircuit to adjust the bobbin in the focusing direction are divided, andthus the weight of the moving portion of the actuator can be reduced,and high sensitivity can be achieved.

Further, the objective lens for a high density optical disc, having ashorter working distance, is installed in the bobbin satisfying Equation1 supra, and thus interference between the objective lens for a highdensity disc and the optical disc can be prevented when the high densityoptical disc is initially installed or a low density optical disc isinstalled.

Furthermore, the actuator used with an optical pickup according to theembodiments of the present invention has a structure in which theobjective lens, having a longer working distance, is installed at adeeper position from the upper side of the bobbin than the objectivelens having a shorter working distance, and thus can contribute to makethe optical pickup thinner.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. An actuator used with an optical pickup, the actuator comprising: alens holder having a plurality of installation holes supporting aplurality of objective lenses to record and/or reproduce data on opticaldiscs having different recording densities; and a magnetic circuit whichdrives the lens holder in focusing and tracking directions, the magneticcircuit being divided into first magnetic circuits located at oppositesides of the lens holder to drive the objective lenses in the focusingdirection, and second magnetic circuits located at opposite sides of thelens holder to drive the objective lenses in the tracking direction,wherein each of the first magnetic circuits comprises a focusing coiland a focusing magnet and each of the second magnetic circuits comprisesa tracking coil and a tracking magnet; wherein the tracking magnets andthe tracking coils of the second magnetic circuits are arranged closerto a center of the lens holder than the focusing coils and the focusingmagnets of the first magnetic circuits.
 2. The actuator of claim 1,wherein the plurality of installation holes are arranged in a directioncorresponding to the direction of a radius of the optical discs.
 3. Theactuator of claim 1, wherein the focusing magnet is a two-polemagnetized polarization magnet, the poles being disposed in the focusingdirection, and the focusing coil corresponds to the focusing magnet. 4.The actuator of claim 3, wherein the focusing coil is of a rectangularshape in which both long sides of the focusing coil are positioned tocontribute to generation of an electromagnetic force.
 5. The actuator ofclaim 1, wherein the tracking magnet is a two-pole magnetizedpolarization magnet, the poles being disposed in the tracking direction,and the tracking coil corresponds to the tracking magnet.
 6. Theactuator of claim 5, wherein the tracking coil is of a rectangular shapein which both long sides of the tracking coil are positioned tocontribute to generation of an electromagnetic force.
 7. The actuator ofclaim 1, wherein the tracking magnet is a three-pole magnetizedpolarization magnet, the poles being disposed in the tracking direction,and two tracking coils corresponding to the tracking magnet.
 8. Theactuator of claim 7, wherein the tracking coils are of a rectangularshape in which both long sides of the tracking coils are positioned tocontribute to generation of an electromagnetic force.
 9. The actuator ofclaim 1, wherein the first magnetic circuits further include at leastone yoke of an internal yoke and an external yoke to guide magnetic fluxgenerated in the focusing magnet.
 10. The actuator of claim 9, whereinthe internal yoke is used as a mount to install the tracking magnet. 11.The actuator of claim 1, wherein the plurality of installation holesinclude a first installation hole in which a first objective lens for atleast one type of low density optical disc is installed, and a secondinstallation hole in which a second objective lens for a high densityoptical disc is installed.
 12. The actuator of claim 11, wherein thesecond installation hole is located in a more inner diameter of theoptical disc than the first installation hole.
 13. The actuator of claim12, wherein the low density optical disc is at least one selected from aCD-family optical disc and a DVD-family optical disc, and the highdensity optical disc has a higher density than the DVD-family opticaldisc.
 14. An actuator used with an optical pickup, the actuatorcomprising: a lens holder having a plurality of installation holessupporting a plurality of objective lenses to record and/or reproducedata on optical discs having different recording densities; and firstand second magnetic circuits located adjacent to the lens holder todrive the lens holder in a tracking direction, and third and fourthmagnetic circuits, located adjacent to the first and second magneticcircuits, respectively to drive the lens holder in a focusing direction,wherein the first and second magnetic circuits each include a trackingmagnet and a tracking coil and each of the third and fourth magneticcircuits each include a focusing magnet and a focusing coil wherein thetracking magnets and the tracking coils of the first and second magneticcircuits are arranged closer to a center of the lens holder than thefocusing coils and the focusing magnets of the third and fourth magneticcircuits.